The invention is directed to a material for electrical contacts of silver, tin oxide and one or more additional metal oxides.
For the production of electrical contact pieces, e.g., switches, for a large number of uses previously silver-cadmium oxide has proven best. However, because of the load placed on the environment by cadmium oxide (CdO) there have been intensive efforts to replace CdO by another metal oxide. In these investigations it has been found that tin oxide (SnO.sub.2) is a suitable replacement for CdO. Because of the higher thermal stability of SnO.sub.2 compared to CdO there results a clearly reduced rate of consumption which leads to a longer working life in the switching device. A very substantial disadvantage of Ag/SnO.sub.2 -contacts, however, is that the contact resistance at the contact is too high after several thousand switchings due to the formation of a coating layer. This as a rule this leads to increased temperatures (excess temperatures) in the switching device, which can lead to the destruction of the device and therefore are inadmissible.
A further disadvantage of these Ag/SnO.sub.2 materials compared to Ag/CdO materials is in the lower margin of safety against welding. The strengths which are required to break bridge welds are in part twice as high as those with Ag/CdO contacts. Consequently, there is the danger of switching disturbances in employing Ag/SnO.sub.2.
Therefore, there have been attempts by the addition of further metal oxides to Ag/SnO.sub.2 to increase the margin of safety in welding, whereby for example bismuth oxide (German OS No. 27 54 335) or indium oxide (German OS 24 28 147) are used. Indeed, these additives improve the margin of safety in welding but create an increased temperature at the contact and at the switching device which impairs the working life of the device.
By addition of tungsten oxide (WO.sub.3) to Ag/SnO.sub.2 there can be attained the reduction of both the excess temperature and the welding strength compared to Ag/SnO.sub.2 (German OS 29 33 338 and related Bohm U.S. application Ser. No. 174,827 filed Aug. 4, 1980 and now U.S. Pat. No. 4,330,330). Through this there is attained a contact material comparable with Ag/CdO which even reaches a considerably higher working life. (The entire disclosure of the Bohm U.S. application is hereby incorporated by reference and relied upon.) However, there are desirable contact materials which have still lower tendencies to weld and lower excess temperatures.
It was the task of the invention to provide a material for electrical contacts made of silver, tin oxide and one or more additional metal oxides which has a high working life, a still lower tendency to weld and still lower temperatures in the switches compared to the known contact materials.